Messages in this thread | | | Date | Wed, 12 Feb 2014 19:28:53 -0500 | From | Tejun Heo <> | Subject | Re: [PATCH 2/2] cgroup: bring back kill_cnt to order css destruction |
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Hello, Hugh.
On Wed, Feb 12, 2014 at 03:06:26PM -0800, Hugh Dickins wrote: > Sometimes the cleanup after memcg hierarchy testing gets stuck in > mem_cgroup_reparent_charges(), unable to bring non-kmem usage down to 0. > > There may turn out to be several causes, but a major cause is this: the > workitem to offline parent can get run before workitem to offline child; > parent's mem_cgroup_reparent_charges() circles around waiting for the > child's pages to be reparented to its lrus, but it's holding cgroup_mutex > which prevents the child from reaching its mem_cgroup_reparent_charges(). > > Further testing showed that an ordered workqueue for cgroup_destroy_wq > is not always good enough: percpu_ref_kill_and_confirm's call_rcu_sched > stage on the way can mess up the order before reaching the workqueue. > > Instead bring back v3.11's css kill_cnt, repurposing it to make sure > that offline_css() is not called for parent before it has been called > for all children. > > Fixes: e5fca243abae ("cgroup: use a dedicated workqueue for cgroup destruction") > Signed-off-by: Hugh Dickins <hughd@google.com> > Reviewed-by: Filipe Brandenburger <filbranden@google.com> > Cc: stable@vger.kernel.org # v3.10+ (but will need extra care) > --- > This is an alternative to Filipe's 1/2: there's no need for both, > but each has its merits. I prefer Filipe's, which is much easier to > understand: this one made more sense in v3.11, when it was just a matter > of extending the use of css_kill_cnt; but might be preferred if offlining > children before parent is thought to be a good idea generally.
Not that your implementation is bad or anything but the patch itself somehow makes me cringe a bit. It's probably just because it has to add to the already overly complicated offline path. Guaranteeing strict offline ordering might be a good idea but at least for the immediate bug fix, I agree that the memcg specific fix seems better suited. Let's apply that one and reconsider this one if it turns out we do need strict offline reordering.
Thanks a lot!
-- tejun
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